The present invention relates to electrically-conductive contrast-improving (CI) filters made of multi-layer thin film coatings. Contrast-improving filters are often used to enhance the image contrast and reduce glare from the screen of a video display terminal or a television set. The screen can be a cathode ray tube (CRT), a flat panel display screen or other type of video display device. The CI filter can be placed between the viewer or operator and the screen, or can be directly coated on the screen itself.
The contrast improvement is accomplished by introducing optical absorption between the viewer and the screen. This absorption improves the contrast between the video image created on the screen and the reflected extraneous light. In the case of a contrast-improving filter between the observer and the screen, the light from the video image is transmitted out through the contrast-improving filter and is attenuated by virtue of the absorption of the contrast-improving filter. However, extraneous light from windows or room lights which would ordinarily reflect from the screen and reduce the contrast of the video image must pass through the CI filter twice-once traveling from the light source to the screen, and then again after reflection from the screen to the observer. Because of the absorption in the contrast-improving filter, the extraneous light is attenuated twice and its intensity is reduced by the square of the transmission of the filter, while the light from the video image is only attenuated once. For example, assuming little or no single surface reflectance for the contrast-improving filter, if the contrast-improving filter has a transmission of about 40%, the video image would be reduced to 40% of its original intensity, but the extraneous light would be reduced to 16% of its original intensity because of the double pass through the CI filter.
The glare reduction feature of the contrast-improving filter is accomplished by reducing the single surface reflectance of the surface of the screen from a value of typically 4% to less than a fraction of a percent. Such a coating is typically called an anti-reflection (AR) coating.
A third desirable property of the contrast-improving filter is that it be electrically conductive. For a modest level of film resistivity (above 10,000 ohms/.quadrature.), the static charge of the screen will be eliminated and it will not attract dust. For lower resistivity levels (below 60 ohms/.quadrature.), the film will offer some measure of protection from the effects of electromagnetic interference/radio frequency interference (EMI/RFI).
Glare reduction conductive coatings can be made by incorporating a layer of indium tin oxide (ITO) or similar transparent conductive material into the coating. These coatings must usually be deposited at elevated temperatures to achieve the best properties of the conductive layer.
Alternatively, thin metal layers may be used to provide both the absorption and the conduction required for the filter. The background of Bjornard U.S. Pat. No. 5,091,244 describes an interference filter which includes layers of thin soft metal films such as silver, gold or copper. The metal layers must be thin in order to transmit a sufficient fraction of the visible light. Bjornard states that such layers are not durable, having poor scratch resistance. Additionally, thin films of silver or copper are vulnerable to corrosion and may deteriorate within a few months when used on unprotected surfaces.
Bjornard also discusses the use of titanium nitride to provide conductivity and absorption. However, the absorption in such a film is fairly high (50% or more), and the resistivity is not low enough to provide significant EMI/RFI shielding.